1. Field of the Invention
This invention relates to integrated circuit manufacturing technology and, more specifically, to structures for making low resistance contact through a dielectric layer to a diffusion region in an underlying silicon layer. The structures include an amorphous titanium nitride barrier layer that is deposited via chemical vapor deposition.
2. State of the Art
The compound titanium nitride (TiN) has numerous potential applications because it is extremely hard, chemically inert (although it readily dissolves in hydrofluoric acid), an excellent conductor, possesses optical characteristics similar to those of gold, and has a melting point around 3000xc2x0 C. This durable material has long been used to gild inexpensive jewelry and other art objects. However, during the last ten to twelve years, important uses have been found for TiN in the field of integrated circuit manufacturing. Not only is TiN unaffected by integrated circuit processing temperatures and most reagents, it also functions as an excellent barrier against diffusion of dopants between semiconductor layers. In addition, TiN also makes excellent ohmic contact with other conductive layers.
In a common application for integrated circuit manufacture, a contact opening is etched through an insulative layer down to a diffusion region to which electrical contact is to be made. Titanium metal is then sputtered over the wafer so that the exposed surface of the diffusion region in coated. The titanium metal is eventually converted to titanium silicide, thus providing an excellent conductive interface at the surface of the diffusion region. A titanium nitride barrier layer is then deposited, coating the walls and floor of the contact opening. Chemical vapor deposition of tungsten or polysilicon follows. In the case of tungsten, the titanium nitride layer provides greatly improved adhesion between the walls of the opening and the tungsten metal. In the case of the polysilicon, the titanium nitride layer acts as a barrier against dopant diffusion from the polysilicon layer into the diffusion region.
Titanium nitride films may be created using a variety of processes. Some of those processes are reactive sputtering of a titanium nitride target; annealing of an already deposited titanium layer in a nitrogen ambient; chemical vapor deposition at high temperature and at atmospheric pressure, using titanium tetrachloride, nitrogen and hydrogen as reactants; and chemical vapor deposition at low-temperature and at atmospheric pressure, using ammonia and Ti(NR2)4 compounds as precursors. Each of these processes has its associated problems.
Both reactive sputtering and nitrogen ambient annealing of deposited titanium result in films having poor step coverage, which are not useable in submicron processes. Chemical vapor deposition processes have an important advantage in that conformal layers of any thickness may be deposited. This is especially advantageous in ultra-large-scale-integration circuits, where minimum feature widths may be smaller than 0.5 xcexcm. Layers as thin as 10 xc3x85 may be readily produced using CVD. However, TiN coatings prepared using the high-temperature APCVD process must be prepared at temperatures between 900-1000xc2x0 C. The high temperatures involved in this process are incompatible with conventional integrated circuit manufacturing processes. Hence, depositions using the APCVD process are restricted to refractory substrates such as tungsten carbide. The low-temperature APCVD, on the other hand, though performed within a temperature range of 100-400xc2x0 C. that is compatible with conventional integrated circuit manufacturing processes, is problematic because the precursor compounds (ammonia and Ti(NR2)4) react spontaneously in the gas phase. Consequently, special precursor delivery systems are required to keep the gases separated during delivery to the reaction chamber. In spite of special delivery systems, the highly spontaneous reaction makes full wafer coverage difficult to achieve. Even when achieved, the deposited films tend to lack uniform conformality, are generally characterized by poor step coverage, and tend to deposit on every surface within the reaction chamber, leading to particle problems.
U.S. Pat. No. 3,807,008, which issued in 1974, suggested that tetrakis dimethylamino titanium, tetrakis diethylamino titanium, or tetrakis diphenylamino titanium might be decomposed within a temperature range of 400-1,200xc2x0 C. to form a coating on titanium-containing substrates. It appears that no experiments were performed to demonstrate the efficacy of the suggestion, nor were any process parameters specifically given. However, it appears that the suggested reaction was to be performed at atmospheric pressure.
In U.S. Pat. No. 5,178,911, issued to R. G. Gordon, et al., a chemical vapor deposition process is disclosed for creating thin, crystalline titanium nitride films using tetrakis-dimethylamido-titanium and ammonia as precursors.
In the J. Appl. Phys. 70(7) October 1991, pp 3,666-3,677, A. Katz and colleagues describe a rapid-thermal, low-pressure, chemical vapor deposition (RTLPCVD) process for depositing titanium nitride films, which, like those deposited by the process of Gordon, et al., are crystalline in structure.
This invention constitutes a contact structure incorporating an amorphous titanium nitride barrier layer formed via low-pressure chemical vapor deposition (LPCVD) utilizing tetrakis-dialkylamido-titanium, Ti(NMe2)4, as the precursor. Although the barrier layer compound is primarily amorphous titanium nitride, its stoichiometry is variable, and it may contain carbon impurities in amounts which are dependent on deposition and post-deposition conditions. The barrier layers so deposited demonstrate excellent step coverage, a high degree of conformality, and an acceptable level of resistivity. Because of their amorphous structure (i.e., having no definite crystalline structure), the titanium nitride layer acts as an exceptional barrier to the migration of ions or atoms from a metal layer on one side of the titanium carbonitride barrier layer to a semiconductor layer on the other side thereof, or as a barrier to the migration of dopants between two different semiconductor layers which are physically separated by the barrier layer.
The contact structure is fabricated by etching a contact opening through a dielectric layer down to a diffusion region to which electrical contact is to be made. Titanium metal is deposited over the surface of the wafer so that the exposed surface of the diffusion region is completely covered by a layer of the metal. Sputtering is the most commonly utilized method of titanium deposition. At least a portion of the titanium metal layer is eventually converted to titanium silicide, thus providing an excellent conductive interface at the surface of the diffusion region. A titanium nitride barrier layer is then deposited using a low-pressure chemical vapor deposition (LPCVD) process, coating the walls and floor of the contact opening. Chemical vapor deposition (CVD) of polycrystalline silicon, or of a metal, such as tungsten, follows, and proceeds until the contact opening is completely filled with either polycrystalline silicon or the metal. In the case of the polysilicon, which must be doped with N-type or P-type impurities to render it conductive, the titanium nitride layer acts as a barrier against dopant diffusion from the polysilicon layer into the diffusion region. In the case of CVD tungsten, the titanium nitride layer protects the junction from reactions with precursor gases during the CVD deposition process, provides greatly improved adhesion between the walls of the opening and the tungsten metal, and prevents the diffusion of tungsten atoms into the diffusion region.
Deposition of the titanium nitride barrier layer takes place in a low-pressure chamber (i.e. a chamber in which pressure has been reduced to less than 100 torr prior to deposition), and utilizes a metal-organic tetrakis-dialkylamido-titanium compound as the sole precursor. Any noble gas, as well as nitrogen or hydrogen, or a mixture of two or more of the foregoing may be used as a carrier for the precursor. The wafer is heated to a temperature within a range of 200-600xc2x0 C. Precursor molecules which contact the heated wafer are pyrolyzed to form titanium nitride containing variable amounts of carbon impurities, which deposits as a highly conformal film on the wafer.
The carbon content of the barrier film may be minimized by utilizing tetrakis-dimethylamido-titanium, Ti(NMe2)4, as the precursor, rather than compounds such as tetrakis-diethylamido-titanium or tetrakis-dibutylamido-titanium, which contain a higher percentage of carbon by weight. The carbon content of the barrier film may be further minimized by performing a rapid thermal anneal step in the presence of ammonia.
The basic deposition process may be enhanced to further reduce the carbon content of the deposited titanium nitride film by introducing one or more halogen gases, or one or more activated species (which may include halogen, NH3, or hydrogen radicals) into the deposition chamber. Halogen gases and activated species attack the alkyl-nitrogen bonds of the primary precursor and convert displaced alkyl groups into volatile compounds.
As heretofore stated, the titanium carbonitride films formed by the instant chemical vapor deposition process are principally amorphous compounds. Other processes currently in use for depositing titanium nitride-containing compounds as barrier layers within integrated circuits result in titanium nitride having crystalline structures. As atomic and ionic migration tends to occur at crystal grain boundaries, an amorphous film is a superior barrier to such migration.